The enantinomers of chlormezanone (1) may be achieved by enantioselective HPLC separation with a yield of 98% using a OD-Daicel column. Both enantiomers bind to human serum albumin (HSA) at pH 7.4 to a range of 11-12%. Binding to the globuline fractions is much less (2-4%, equilibrium dialysis, validation by ultrafiltration). It could be demonstrated by means of 1H-NMR spectroscopy that 1 binds to HSA with the benzene ring as well as with the thiazanone ring. The velocity of racemization could be measured for the first time using a BSA column. The enantiomers undergo racemization at pH 7.4 and 37 degrees C with a halflife of approx. 20.5 h.